> What does it normally cost to get a machine
> shop to lighten a stock flywheel? Are there
> other good sources for such?
A machine shop will likely charge you a heck of a lot
less to lighten your stock flywheel, but it will
probably be nowhere near as light as an alloy
flywheel. I'm not sure what the exact difference in
weight of the stock flywheel is vs. the Moss alloy
version, but you can't get THAT much weight out of a
stock flywheel by lightenting it yourself, or having a
machine shop do it for you. Then again, that might
not be such a bad thing...
> Also, what are the downsides to a lightened
> flywheel? Harder to idle?
Not really. It might idle a little less smoothly, and
it might occasionally be more prone to stalling if you
shock it - like locking the sheels in a panic maneuver
or soemthing and forgetting to put the clutch in, but
the big difference is in response. A lightened
flywheel means there is less stored reciprocating mass
that the engine has to accelerate, so a light flywheel
makes the car respond to the throttle more rapidly.
It also makes the car accelerate through its RPM range
more quickly. A good before and after test would be
to accelerate from 2000 to 4500 RPM in third gear (or
any other rev range you feel is critical to you). As
long as the rev range involves fairly high RPMs, your
car will accelerate through the range more quickly
with the light flywheel.
The downside is that since less mass is stored in the
flywheel, low RPM torque is reduced (or at least the
impression of low RPM torque is reduced). For street
driving, where low RPM resonse, smoothness and torque
are perhaps paramount, it doesn't pay to go too light
on the flywheel. But in a car that sees mostly
agressive, higher RPM use, you'll get some significant
benefits.
> Likewise, what are the quantifiable
> differences. If my 130 hp MGB will turn a
> 2:20 lap with a stock flywheel, what should
> I expect to see with a lightened one?
You'll be down under two minutes, easy. Hah! Just
kidding! Seriously, I can't imagine that anyone would
be able to answer this with anything other than a WILD
guess. There are simply too many variable at stake.
In my opinion, a light flywheel is super fun and cool
in a competition car or very agressively-driven street
car. Matching revs is a breeze, because you blip the
throttle and WHAM! The engine responds super-quick.
But in a street car, I'd probably spend my money
elsewhere (unless I already had... 130 BHP is nothing
to sneeze at in a B!)
Hope that helps,
=====
Paul Misencik
1971 MGB Tourer - Blaze
Huntersville, NC USA
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